1. Field of the Invention
The invention relates to an integrated vertical silicon carbide PN power diode and a circuit arrangement with such power diodes.
Semiconductor components are at present made from a semiconductor material such as silicon or also from gallium arsenide (GaA) and gallium phosphate (Ga3P4), which, however, have a low thermal, chemical and physical stability.
Silicon carbide (SiC) on the other hand is a semiconductor material that has a physically highly stable crystal structure particularly due to its wurtzite- or zinc-blende crystal lattice. Depending on the polytype, SiC monocrystals have a large energy band gap of 2.2 eV to 3.3 eV, which makes them thermally, and especially mechanically, particularly stable and resistant to radiation damage. This makes SiC very attractive for such semiconductor components, which are exposed to extreme temperatures or extreme operating or environmental conditions, such as prevail, for example, in automotive and railway engineering. Semiconductor components based on SiC are able to operate over a large voltage and temperature range, for example up to 650° C. to 800° C., have very good switching properties and low losses and can also be operated at very high working frequencies. Compared to silicon, SiC has a stronger breakdown field (up to 10 times higher than for silicon) due to the better material properties, a higher heat conductivity (more than three times higher than with silicon) and a larger energy band gap (2.9 eV for 6 H—SiC).
SiC is particularly suitable for power components with a very high blocking voltage (600 V up to a few kV), such as high voltage (switching) diodes and field-effect transistors. Such SiC semiconductor components are, for example, used in converters for electrical drives, in switched-mode power supplies or in uninterruptible power supply systems. The purpose of the use of higher operating voltages is usually to be able to convert larger electrical outputs (of a magnitude of some kilowatts) with the same current.
Because the SiC semiconductor technology is still relatively young and in many respects not yet optimized, a series of problems exist in the production of SiC-based semiconductor components, which are yet to be solved to enable SiC components to become a reality in many component variants and in large unit quantities. The particular reason for this is that the same processes used for silicon components cannot simply be used for the production of SiC components. For example, doping by diffusion is all but unrealizable for SiC. Furthermore, the electrical activation of the doping atoms applied during the ion implantation is relatively difficult to control with SiC. For the aforementioned reasons, SiC is at present advantageously used for semiconductor components that can be produced by relatively simple technology such as Schottky diodes, PN diodes and field-effect transistors.
To explain the general problem, FIG. 1 of the drawing shows a schematic part section of the construction of an SiC power diode. The SiC power diode with the reference character 1 contains a very thick N-doped SiC semiconductor substrate, which is, for example, part of an SiC semiconductor wafer. The back of the semiconductor substrate 2 is connected to a cathode terminal K. An N-doped buffer layer 3, a low N-doped drift zone 4 and a highly P-doped emitter zone 5, which is connected on the front to an anode terminal A, are mounted in series on the front of the SiC semiconductor substrate 2. The thickness of the drift zone 4 and its doping concentration essentially determines the blocking state characteristics of the power diode 1.
FIG. 1a shows an idealized form of a current/voltage curve of the SiC power diode from FIG. 1 relative to temperature T, with, on the abscissa, the forward voltage UF being shown and, on the ordinate, the current I flowing at the same time being shown. KP is the intersection point of both diode characteristic curves at high currents or high forward voltages. The working point normally used for an SiC power diode is typically below the intersection point KP of the diode characteristic curves.
FIG. 1a shows that the forward voltage UF reduces with increasing temperature T at a given injected current I. At a given impressed forward voltage UF, the diode current I increases with increasing temperature T. This phenomenon is also known as negative temperature coefficients (du/dt<0) at constant current. The diodes normally used e.g. silicon diodes, on the other hand have a positive temperature coefficient with the forward voltage UF also increasing at a constant diode current I with increasing temperature T.
The phenomenon of the negative temperature coefficient is on the one hand due to the service life of a minority charge carrier increasing with increasing temperature. In addition or alternatively, this phenomenon is also due to the contact resistance between the anode metalling and the highly P-doped emitter zone reducing with increasing temperature.
This particular phenomenon with an SiC SC power diode as described in FIG. 1a, i.e. the presence of a negative temperature coefficient, is unwelcome or even damaging, particularly where several SiC power diodes are connected in parallel because, due to technology-imposed differences between different power diodes, a uniform distribution of the total current to the various power diodes of the parallel circuit cannot usually be guaranteed. As a result, one of the power diodes of the parallel circuit therefore typically takes a higher diode current than the other power diodes, which has the direct effect of causing this power diode to suffer a greater increase in temperature than the other power diodes. Because of the negative temperature coefficient, this in turn causes the current even through this power diode to additionally increase due to the characteristic curve, thus leading to a further increase in temperature of this power diode.
This latter phenomenon typically quickly leads to failure of this power diode and thus of the complete diode parallel circuit. This is a condition that should, of course, be avoided.